Investigation of Hydrogenation of Formic Acid to Methanol using H2 or Formic Acid as a Hydrogen Source

Article abstract of DOI:10.1021/acscatal.6b03194

Production of methanol (MeOH) from CO₂ is strongly desired as a key chemical feedstock and a fuel. However, the conventional process requires elevated temperature and pressure, and high temperature restricts the productivity of MeOH due to equilibrium limitations between CO₂ and MeOH. This paper describes the efficient hydrogenation/disproportionation of formic acid (FA) to MeOH by using iridium catalysts with electronically tuned ligands and by optimizing reaction conditions. An iridium complex bearing 5,5′-dimethyl-2,2′-bipyridine in FA hydrogenation achieved MeOH selectivity with H₂ of up to 47.1% for FA hydrogenation under 4.5 MPa of H₂ in the presence of H₂SO₄. The final concentration of MeOH of 3.9 M and a TON of 1314 were obtained in 12 M FA aqueous solution including 10 mol % of H₂SO₄ at 60°C under 5.2 MPa of H₂. Even under atmospheric pressure without introduction of external hydrogen gas, the FA disproportionation under deuterated conditions produced MeOH with 15.4% selectivity. Furthermore, the isotope effect and NMR studies revealed mechanistic insight into the catalytic hydrogenation of FA to MeOH. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.6b03194

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Article
Investigation of Hydrogenation of Formic Acid to
2
Methanol using H or Formic Acid as a Hydrogen Source
Akihiro Tsurusaki, Kazuhisa Murata, Naoya Onishi, Katerina Sordakis, Gabor Laurenczy, and Yuichiro Himeda
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03194 • Publication Date (Web): 22 Dec 2016
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Methanol using H or Formic Acid as a Hydrogen
2
Source
Akihiro Tsurusaki,†,§ Kazuhisa Murata, Naoya Onishi, Katerina Sordakis, Gábor Laurenczy,
†
†
‡
‡,*
and Yuichiro Himeda†,*
†National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1
Higashi, Tsukuba, Ibaraki, 305-8565, Japan
‡
Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne
(EPFL), Avenue Forel 2, 1015 Lausanne, Switzerland
ABSTRACT
Production of methanol (MeOH) from CO
fuel. However, the conventional process requires elevated temperature and pressure, and high
temperature restricts the productivity of MeOH due to equilibrium limitation between CO and
2
is strongly desired as a key chemical feedstock and a
2
MeOH. This paper describes the efficient hydrogenation/disproportionation of formic acid (FA)
to MeOH by using the iridium catalysts with electronic-tuned ligands and by optimizing reaction
conditions. The iridium complex bearing 5,5′-dimethyl-2,2′-bipyridine in FA hydrogenation
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achieved the MeOH selectivity with H
in the presence of H SO
obtained in 12 M FA aqueous solution including 10 mol% H
2
up to 47.1% for FA hydrogenation under 4.5 MPa of H
2
2
4
. The final concentration of MeOH of 3.9 M and TON of 1314 was
SO at 60 ºC under 5.2 MPa of H .
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Even under the atmospheric pressure without introduction of external hydrogen gas, the FA
disproportionation in deuterated conditions produced MeOH with 15.4% selectivity. Furthermore,
the isotope effect and NMR studies revealed the mechanistic insight of the catalytic hydrogenation
of FA to MeOH.
KEYWORDS: hydrogenation of formic acid, disproportionation of formic acid, methanol
production, dehydrogenation of formic acid, iridium complexes,
INTRODUCTION
Methanol (MeOH), which is widely available for the production of many chemicals and fuel, is
produced from syngas in industrial productions. In addition, it has been suggested as an energy
1
,2
carrier (12.6 wt% hydrogen content) in the “methanol economy” in recent years. CO
MeOH can be converted into each other by the hydrogenation/dehydrogenation via formic acid
FA) and formaldehyde according to the chemical equation (Scheme 1). Recently, several
2
and
(
research groups intensively investigated the hydrogen production by dehydrogenation of
MeOH.3-12 Although the direct hydrogenation of CO
by H to MeOH is the most straightforward
2
2
process (eq. 1), it requires energy-intensive conditions such as high temperature (200-250 °C)
and/or high pressure (5-10 MPa), because of the extreme thermodynamic stability of CO . In
addition, the conversion of MeOH from CO is potentially equilibrium-limited by a high
2
2
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3-15
temperature because of the negative ∆H and ∆S of this process.
Exceptionally, George Olah
to
million litres of MeOH by direct hydrogenation in 2015, by using the specific regional energy
Renewable Methanol Plant, which went into operation in 2012, converted 5500 tonnes of CO
2
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supply in Iceland. However, the MeOH production that can work at a low temperature is highly
desired in most cases, to avoid the equilibrium limitation and minimization of energy
consumption.13
H2
H O
2
H2
H2
CO2
HCO H
HCHO
CH OH
3
2
H2
H
2
H
2
O
H2
Scheme 1. Interconversion between CO , FA, and MeOH.
2
Lately, the production of MeOH from CO
2
using homogenous catalysts has received
considerable attention.12,17-19 In 1993, Sasaki reported production of MeOH by CO2
2
0
hydrogenation using Ru
3
(CO)12-KI in N-methyl-2-pyrrolidone at 240 °C and 9 MPa. Recently,
Sanford and Leitner have demonstrated the successful example of cascade/tandem catalysis,
which involves (i) hydrogenation of CO
2
to FA, (ii) esterification to formate ester, and (iii)
hydrogenation of the ester to MeOH using three compatible catalysts in each steps.21,22 It was
2
3
also found that the MeOH production was promoted by just adding acid. Very recently, it was
reported that the modified catalysis using amine led to improvement of productivity.24,25 Sanford
and Milstein independently reported the indirect hydrogenation of the CO
2
to MeOH via
derivatives, such as carbonate,26,27 carbamates, formates, and urea,
28
26
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hydrogenation of CO
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which can be easily available from CO
2
at relatively milder reaction conditions. Even through
the cutting-edge catalytic systems, elevated reaction temperature (> 100 °C) has been still
required.
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In the previous communication, we have reported the direct CO
2
hydrogenation to MeOH
using the proton-responsive iridium catalyst, [Cp*Ir(4DHBP)(H
2
O)](SO ) (1a, 4DHBP = 4,4′-
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dihydroxy-2,2′-bipyridine) under acidic conditions. In that study, it was clarified that the
addition of sulfuric acid (H
2
SO
4
) remarkably improved the MeOH productivity in the catalysis.30
However, there is still room for improvement of the catalysis, which consists of hydrogenation of
CO
2
to FA and hydrogenation of FA to MeOH. The hydrogenation of CO
2
to FA (not formate)
3
1
32
33
without base (eq. 2), which has been reported by Ogo, Laurenczy, and Li , is strongly
governed by the equilibrium. Furthermore, the following hydrogenation of FA (eq. 3) competes
with dehydrogenation of FA as a reverse reaction of hydrogenation of CO , while several
2
efficient homogeneous catalysts for hydrogenation of acid were reported recently.34-39
+
2
Me
N
Cp*
2–
Ru
+
Ir
(OTf)
2
H O
2
N
Me
I
PPh2
PPh2
PPh2
Mo
OC
H
II
OC CO
III
Chart 1. Homogeneous catalysts for the FA disproportionation
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CO
2
(g) + 3 H
2
(g) → CH
3
OH (l) + H
2
O (l)
(eq. 1)
(eq. 2)
(eq. 3)
(eq. 4)
H
2
(g) + CO
2
(g) → HCO H (aq)
2
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HCO
2
H (l) + 2 H
2
(g) → CH
3
OH (l) + H O (l)
2
3 HCO
2
H(aq) → CH
3
OH (aq) + H
2
O (aq) + 2 CO (g)
2
However, the disproportionation of FA (i.e., the hydrogenation of FA to MeOH using
hydrogen source from dehydrogenation of FA) is also one of the promising methods for the
indirect production of MeOH from CO
2
(eq. 4),40-42 because formate production by
hydrogenation of CO
2
have been significantly improved in the last two decades.43-46 Although
the FA disproportionation was previously regarded as the side reaction of dehydrogenation of
4
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FA, the possibility of the MeOH production by the reaction catalysed by iridium catalyst I was
re-recognised by Goldberg and co-workers in 2013 (Chart 1). The maximum values of turnover
1
−
number (TON) and turnover frequency (TOF) were 156 and 6.5 h , respectively, in 12 M FA
aqueous solution at 80 °C for 24 h in a closed vessel. In the system, MeOH was produced with
only 12% selectivity, while residual FA was dehydrogenated to H
2
and CO
2
in a 12 M FA
4
0
aqueous solution at 60 °C in a closed vessel. Cantat significantly improved the MeOH
selectivity of 50.2% with a ruthenium catalyst and CH
3
C(CH
2
PPh
2
3
) (II), in THF at 150 °C in a
4
1
sealed vessel for 1 h in the presence of methanesulfonic acid, whereas Neary and Parkin
reported the MeOH selectivity of 21% in benzene at 100 °C with the molybdenum catalyst III.42
However, the improvement of MeOH selectivity and the necessity of high temperature are still
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major problems. Thus, development of efficient catalysts are highly desirable from the point of
view of not only the mitigation of the energy consumption but also thermodynamics.
In here, we comprehensively investigated FA hydrogenation/disproportionation to MeOH, using
the iridium catalyst with substituted 2,2′-bipyridine derivatives under various reaction conditions,
since the substituent and positional effect of the bipyridine ligand were not sufficiently
clarified.48,49 We found that the iridium catalyst bearing 5,5′-dimethyl-2,2′-bipyridine, which is
commonplace but has never been tried, showed the high MeOH selectivity for FA
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hydrogenation/disproportionation. In addition, the isotope effect and NMR studies revealed the
mechanistic insight of FA disproportionation to MeOH. It was elucidated by the NMR studies
that the abundance ratio of the intermediates, such as iridium-hydride or iridium-dihydrogen,
depended on H
2
SO
4
and H
2
. Furthermore, MeOH selectivity of 15.4% was obtained even under
D including 5% D SO (0.05 M as solution) in D
the atmospheric pressure in 4 M DCO
2
2
4
2
O
without introduction of hydrogen. The results provided further motivation for the improvement
of selectivity of MeOH production from FA.
Results and Discussion
Ligand Effect for FA hydrogenation
We investigated the catalytic activities of Cp* iridium aqua complexes with various 2,2′-
bipyridine ligands (Chart 2), which were prepared according to an usual procedure (See
Experimental Section in Supporting Information). The hydrogenation of FA to methanol was
carried out under a constant pressure of mixture gases (H
mmol) in H O including 10 mol% H SO (1.6 mmol, 0.4 M as solution: where the term of ‘X
mol% H SO ’ is expressed as a percentage of FA) and a catalyst (5 µmol) with 1500 rpm at 50
2
/CO
2
(1/1)) in 4 M FA (4 mL, 16
2
2
4
2
4
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ºC for 20 h in a stainless autoclave. Initially, a mixed gases of H
2
/CO (1/1) was charged, and the
2
pressure was kept at 3.0 MPa during the course of the reaction by equipped with a back-pressure
regulating valve in order to exclude the effect on the increasing H /CO gases by the FA
dehydrogenation. MeOH and methyl formate were observed by the GC analysis after the reaction
2
2
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because the equilibrium exists between FA/MeOH and methyl formate/H O in acidic conditions
2
50
in water. In all reactions, the term, “the MeOH yield”, expresses the sum of MeOH and methyl
formate in the manuscript. On the other hand, formaldehyde, which is considered as one of the
possible reduction products of FA, was not detected. The result was consistent with that of the
previous report, because the hydrogenation of formaldehyde to MeOH was much faster than that
4
0
of FA to formaldehyde. We note that the MeOH selectivity in the case of the introduction of
external H was calculated as the moles of the produced MeOH divided the amount of the
2
consumed FA, because the initially introduced H
2
gas can serve as the reducing agent of FA (See
Supporting Information).
2
+
R
R
2+
HO
Ir
2+
N
N
Cp*
N
N
N
N
Cp*
Cp*
Ir
SO42-
SO42-
OH
SO42-
Ir
H O
OH
2
H O
2
H O
2
HO
σ
+
p
1a: R = OH
= -0.91)
= -0.78)
= -0.31)
(
σ
+
+
2
1
1
1
1
b: R = OMe
3
(
p
σ
c: R = Me
(
p
σ
+
d: R = H
= 0)
σ
= 0.42)
(
p
(
p
+
e: R = CO H
2
Me
R
Me
+
2+
2+
Me
N
N
Cp*
N
N
-
Cp*
Cp*
Ir
Cl
SO42-
2-
Ir
Ir
SO
4
Cl
H O
N
H O
N
2
2
Me
Me
R
a: R = OH
Me
4
6
5
4b: R = Me
Chart 2. Iridium complexes for FA hydrogenation
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Table 1. Ligand Effect in FA Hydrogenationa
b
−1 b
Entry Cat. Yield of MeOH MeOH Selectivity TON
TOF (h ) FA Conv.
b,c
(%)b
(%)d
(µmol)
with H
2
0
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1a
1b
1c
1d
1e
2
170
1.3
34.1
45.0
71.5
38.3
16.3
7.4
1.7
2.2
3.6
1.9
0.8
0.4
2.5
2.4
3.4
2.9
0.8
0.7
1.3
0.3
80
69
35
12
<1
59
28
14
21
38
14
5
225
2.0
358
6.5
191
10.3
57.9
0.4
81.7
36.9
247
3
5.5
49.3
48.9
68.7
57.7
15.0
14.6
27.0
5.0
4a
4b
5
245
11.0
10.5
4.8
343
0
289
1e
2
75.2
73.1
135
3.3
4b
6
9.2
3f
4g
11.6
17.8
7
4b
4b
25.1
<1
aReaction conditions: 4 M FA (4 mL, 16 mmol) in H
O including 10% H
SO
(1.6 mmol) and a
2
2
4
b
catalyst (5 µmol) was stirred with 1500 rpm at 50 ºC for 20 h under 3.0 MPa of CO
Sum of MeOH and HCO Me was used. The selectivity with H
produced MeOH and HCO Me divided by the mole of the consumed FA. Determined by GC.
2
/H
2
(1/1).
2
2
is calculated from the mole of the
c
2
d
e
f
g
Determined by HPLC. The absence of H
was added.
2
SO . 1.1 equiv. of NaCl was added. 5.0 equiv. of NaCl
4
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Figure 1. Plot of MeOH yields (red), conversion of FA (black), and MeOH selectivity (blue)
+
versus σ
P
values for FA hydrogenation under 3.0 MPa of CO
2
/H (1/1).
2
At first, we examined the electronic effect of 4,4′-positioned substituents in 2,2′-bipyridine on
4
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FA hydrogenation (Table 1). The catalyst 1a that we previously reported gave 170 µmol of
MeOH with only 1.3% selectivity under these reaction conditions (entry 1). Figure 1 shows that
+
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the MeOH selectivity became higher as the σ value increased, while the conversion of FA
P
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became lower. As consequence, the moderated electron-donating methyl complex 1c provided
high yield of MeOH (358 µmol), which depended on the multiplication of the MeOH selectivity
(6.5%) and the conversion of FA (35%) (entry 3). Next, the positional effect of hydroxyl groups
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of a bipyridine ligand was examined. The MeOH yield and selectivity of 2 at 6,6′-position
significantly decreased compared to those of 1a, irrespective of the moderate FA conversion
(entry 6). This lower activity of 2 will be due to the faster protonation of iridium−hydride
intermediate to generate H
2
than the reduction of FA. We have already revealed that the rate-
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determining step of 2 is β-hydrogen elimination of the formato complex to generate
iridium−hydride, whereas that of 1a was H production by the reaction of iridium-hydride with
2
proton.54,55 In contrast, catalysts 3 and 4a at 3,3′- and 5,5′-positions, respectively, significantly
improved both the MeOH yields (1.5 times as large as that of 1a) and selectivities (entries 7 and
8 vs. 1). Especially, 4a showed the highest MeOH selectivity of 11.0% due to lower FA
conversion. A σ-withdrawing group of the hydroxyl group at 3- or 5- (meta) position in contrast
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
to a π-donating group at 4- (para) and 6- (ortho) positions may affect the high MeOH production.
The results of the high MeOH yield with methyl groups (1c) and the high MeOH selectivity at
5
,5′-position (4a) prompted us to investigate the complex 4b with methyl groups at 5,5′-
56
position. To our delight, the MeOH yield increased from 245 µmol (4a) to 343 µmol (4b) with
the comparable MeOH selectivity (entry 9). We also investigated the iridium complex 5 bearing
4
,4′,5,5′-tetramethyl-2,2′-bipyridine, but both the MeOH yield (289 µmol) and selectivity (4.8%)
were lower than those of 1c and 4b (entry 10). As expected, the MeOH yield and selectivity
considerably decreased in the absence of H SO (entry 11, vide infra). Curiously, the chloride
analog 6 instead of an aqua ligand remarkably decreased the yield of MeOH (entry 12), contrary
to the previous report by Goldberg and co-workers, that [Cp*Ir(bpy)(H O)](OTf) and 1:2
and bpy afford the comparable result. On the other hand, Kubiak
2
4
2
2
4
0
mixture of [Cp*IrCl
]
2 2
reported that the corresponding iridium chloride complex of 1c showed poor catalytic activity.49
When NaCl (1.1 eq or 5 eq) was added to the reaction mixture of 4b, the catalytic activities
gradually decreased (entries 13 and 14). These results imply that the strong coordination of
5
7
chloride to iridium center inhibits the generation of the catalytic active species. We also noted
that the analysis of the evolved gases collected after the reaction by using 4b showed no
detectable CO (less than 10 ppm) by the GC (Figure S7).
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Effect on the Reaction Conditions for the FA Hydrogenation
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1
) Pressure. We examined the dependence of the pressure from 0 MPa to 4.5 MPa under the
mixed gases of CO /H (1/1) by using a back-pressure regulating valve in order to keep the
pressure during the course of the reaction (Table 2). These results clearly showed that the yield
of MeOH rose as the increasing partial pressure of H (Figure 2), although the conversion of FA
slightly decreased (Figure S8). The MeOH selectivity of 13.7% under 3.0 MPa with the CO /H
(1/3) was comparable to that under the condition of 4.5 MPa of CO /H (1/1) (entries 5 vs. 6).
The highest MeOH yield (573 µmol) and selectivity (16.0%) were obtained under 3.0 MPa of H
(entry 7). Under CO or argon, the yield of MeOH decreased at the similar level of that without
pressurization (entries 8 and 9 vs 1). Without external H , MeOH selectivity is calculated from
the mole of the produced MeOH and HCO Me divided by the one third of the mole of the
consumed FA. As a consequence, the yield of MeOH was proportional to the partial pressure of
2
2
2
2
2
2
2
2
2
2
2
. This is consistent with the result reported by Goldberg and co-workers.40
H
2
Table 2. Effect on the Pressure of Gasses in FA Hydrogenationa
b
Entry Pressure Yield of MeOH MeOH selectivity TON
TOF
FA Conv.
b,c
b,d
−1 b
e
(
MPa)
(µmol)
with/without H
2
(h )
1.1
2.0
2.7
3.4
4.4
(%)
25
24
24
21
20
1
2
3
4
5
0
114
(8.4)
5.2
22.8
39.3
54.0
68.7
87.9
0.75
1.5
3.0
4.5
197
270
7.2
343
10.5
13.9
439
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1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
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3
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3
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5
5
5
5
5
6
6f
3.0
3.0
3.0
3.0
421
573
194
131
13.7
84.1
115
4.2
5.7
1.9
1.3
19
7g
8h
9i
16.0
22
(13.2)
(10.2)
38.9
26.2
28
24
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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3
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5
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
aReaction conditions: 4 M FA (4 mL, 16 mmol) in H
catalyst 4b (5 µmol) was stirred with 1500 rpm at 50 ºC for 20 h under the gases with pressurization
except for entry 1. Sum of MeOH and HCO
with H is calculated from the mole of the produced MeOH and HCO
the consumed FA. The values in parentheses are MeOH selectivities without H
calculated from the mole of the produced MeOH and HCO Me divided by the one third of the
mole of the consumed FA. Determined by HPLC. The gases of CO
were charged.
O including 10% H
SO
(1.6 mmol) and
2
2
4
b
c
d
2
Me was used. Determined by GC. The selectivity
Me divided by the mole of
, which is
2
2
2
2
e
f
g
h
i
2
/H
2
= 1/3, H
2
, CO , and Ar
2
Figure 2. Plot of MeOH yield versus the partial pressure of H . The number on the plot is the
2
number on entries shown in Table 2.
2
) Concentration of H
2
SO . In the previous paper, we have already reported that the addition of
4
H
2
SO
4
drastically improved the MeOH selectivity of the FA hydrogenation by using 1a, and
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30
H
2
SO
4
showed the highest activities among the seven Brönsted acids. We examined the
dehydrogenation and hydrogenation of FA with the various concentrations of H
2
SO at 60 °C
4
under atmospheric pressure (Table S1, Figure S9). Interestingly, the maximum TOF of the FA
dehydrogenation for 1c and 4b was observed at pH of approximately 1, unlike other Cp*Ir
complexes bearing N,N’-bidentate ligand.52,54,55,58,59 Surprisingly, the MeOH selectivities of 4b
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60
without H exceeded 2 % even under atmospheric pressure, although the distinct change of these
2
was not observed at various pH. On the other hand, the negligible MeOH yield for 1a (< 4
µmol), which is the efficient catalyst for FA dehydrogenation, was obtained under these reaction
conditions (entries 14 and 15 in Table S1).
Under 3.0 MPa of H
without H SO ) to 25% (>50 mol% H
Figure 3). A slight addition of H SO (less than 10 mol%) accelerated not only the MeOH
production, but also the FA conversion. The FA conversion with 2.5 and 5 mol% of H SO
7%, which was twice as large as that without H SO (14%), and gradually decreased with the
increase of H SO concentration. Furthermore, the reaction with the combination of 50% H SO
and 3.0 or 4.5 MPa of H gas afforded 38.0 and 47.1% MeOH selectivity, respectively (entries
2 and 13). The MeOH selectivity of higher than 33.3% indicates that FA, as well as the
2
/CO
2
(1/1), the MeOH selectivity of 4b gradually increased from 3.3
(
2
4
2
SO as FA) with the increase of H SO (Table 3 and
4
2
4
2
4
2
4
was
2
2
4
2
4
2
4
2
1
introduced H2, served as hydrogen donors of the hydrogenation of FA.
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
in FA Hydrogenationa
Table 3. Effect on the Concentration of H
2
SO
4
b
Entry
H
2
SO
4
Yield of MeOH MeOH selectivity TON
TOF
FA Conv.
(%)
b,c
b,d
−1 b
e
(mol% (M)) (µmol)
with H
2
(h )
0.8
2.2
3.2
3.2
3.4
3.5
4.3
4.5
4.8
5.0
5.0
8.1
13.4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0 (0)
75.2
224
318
317
343
351
425
445
485
502
501
808
1340
3.3
15.0
44.9
14
27
27
22
21
13
12
11
12
11
12
13
18
2.5 (0.1)
5 (0.2)
5.1
7.4
63.6
7.5 (0.3)
10 (0.4)
20 (0.8)
35 (1.4)
50 (2.0)
65 (2.6)
80 (3.2)
100 (4.0)
50 (2.0)
50 (2.0)
9.0
63.4
10.5
16.5
21.4
24.7
24.4
27.8
25.8
38.0
47.1
68.7
70.1
85.1
89.0
97.0
1
1
1
1
0
100
1
100
2f
3g
162
268
aReaction conditions: 4 M FA (4 mL, 16 mmol) in H
was stirred with 1500 rpm at 50 ºC for 20 h under 3.0 MPa of CO
O including H
SO
/H (1/1). Sum of MeOH and
is calculated from the mole of
and catalyst 4b (5 µmol)
2
2
4
b
2
2
c
d
HCO
2
Me was used. Determined by GC. The selectivity with H
Me divided by the mole of the consumed FA. Determined by HPLC.
(4.5 MPa) were used instead of CO /H (1/1; 3.0 MPa).
2
e
the produced MeOH and HCO
2
f
g
H
2
(3.0 MPa) and H
2
2
2
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41
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Plot of MeOH yield (blue), the conversion of FA (red), and MeOH selectivity (black)
versus the amount of H
2
SO
4
(mol%) for FA hydrogenation under 3.0 MPa of CO
2
/H (1/1).
2
3
) Initial FA Concentration. Initial concentration of FA also affected the MeOH yield and
selectivity since FA serves as both a hydrogen donor and acceptor. Goldberg and co-workers
reported that TON markedly improved in the high FA concentration, whereas the MeOH
4
0
selectivity slightly increased. In order to examine the effect of the total amount of FA, only
concentration of FA changed while the amount of H SO (1.6 mmol) and the initial volume (4
mL) were kept, as a result, pH values differed respectively. That is, 1 M FA solution (4 mL, 4.0
2
4
mmol) included 40 mol% of H
2
SO
4
(1.6 mmol, 0.4 M), whereas 12 M FA solution (4 mL, 48
(1.6 mmol, 0.4 M). The results in Table 4 and Figure 4
mmol) included 3.3 mol% of H
2
SO
4
clearly showed that the MeOH yield was linearly correlated with the FA concentration, and that
the MeOH selectivity gradually rose as the increase of the FA concentration. The MeOH
selectivity of 14.1% in 12 M FA solution was 2.1 times as large as that of 6.7% in 1 M FA
solution.
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2
2
2
2
3
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3
4
4
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4
4
4
4
4
4
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5
5
5
5
5
5
5
6
Table 4. Effect on the Concentration of FAs in FA Hydrogenationa
Entry FA (M)b
Yield
MeOH
of MeOH
Selectivity
TONc TOF
FA
Consumption
of FA (mmol)
1
c
−
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(h )
Conv.
c,d
c,e
(%)f
(
µmol)
with H
2
1
2
3
4
5
1 (40)
2 (20)
4 (10)
8 (5)
95.9
6.7
19.2 1.0
36
24
21
13
13
1.44
1.95
3.29
4.02
6.26
171
343
545
881
8.7
34.2 1.7
68.7 3.4
10.5
13.5
14.1
109
176
5.4
8.8
12 (3.3)
aReaction conditions: An aqueous FA solution including H
µmol) was stirred with 1500 rpm at 50 ºC for 20 h under 3.0 MPa of CO
SO (1.6 mmol) and catalyst 4b (5
2
4
b
2
/H
Me was used. Determined
is calculated from the mole of the produced MeOH and HCO Me
2
(1/1). A ratio of
c
d
H
2
SO
4
as FA is shown in the parentheses. Sum of MeOH and HCO
2
e
by GC. The selectivity with H
divided by the mole of the consumed FA. Determined by HPLC.
2
2
f
Figure 4. Plot of MeOH selectivity (red) and MeOH yield (blue) versus the concentration of FA
for FA hydrogenation under 3.0 MPa of CO /H (1/1).
2
2
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4
) Catalyst concentration. The yield of MeOH and the conversion of FA were linearly
correlated with the concentration of 4b in the range from 2.5 to 10 µmol (Table S2 and Figure
S10), and the TOF and MeOH selectivity with H were comparable to each other. This result
indicates FA hydrogenation was the first order in iridium complex concentration.
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47
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50
51
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53
54
55
56
57
58
59
60
2
5
) Reaction Temperature. The temperature dependence of the FA hydrogenation was
investigated in the range from 30 °C to 70 °C. Although the yield of MeOH and FA conversion
increased, the MeOH selectivity largely decreased from 15.0 (30 °C) to 6.2% (70 °C), probably
due to the entropic reason (Table S3, Figure 5). It is noteworthy that MeOH was produced with
1
5% selectivity around room temperature (30 °C) similar to our previous paper.30
Figure 5. Plot of MeOH selectivity (red) and MeOH yield (blue) versus temperature for FA
hydrogenation under 3.0 MPa of CO /H (1/1).
2
2
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2
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5
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5
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0
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0
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3
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. Time course of MeOH yield (blue) and MeOH selectivity (red) for FA hydrogenation in
4 M FA solution (10 mL, 40 mmol) including 10% H
under 3.0 MPa of CO /H (1/1).
2
SO (4.0 mmol) with 4b (5 µmol) at 50 ºC
4
2
2
6
) Reaction Time. Time course of MeOH production showed that the yield of MeOH
continuously increased (Figure 6 and Table S4), and the yield of MeOH and TON finally reached
.12 mmol and 424, respectively, after 260 h (entry 7). During the course of the reaction, the
MeOH selectivities were comparable to each other (9.2-11.6%).
2
7
) Further Increasing of the Production of Methanol under Optimized Conditions. In order
to obtain the higher TON and MeOH yield, we carried out the further experiments on the basis of
the fundamental experiments described above (Table S5). The highest TON of 1314 (TOF: 6.9
h−1) was achieved by the reaction of 12 M FA solution including 10% H
SO (12 mmol) at 60 ºC
2
4
for 190 h under 5.2 MPa of H . A 3.9 M of MeOH was obtained with MeOH selectivity of
2
29.9% and 73% conversion of FA (entry 1). On the other hand, 95% conversion of FA with the
MeOH selectivity of 45.5% was achieved by the reaction of 4 M FA solution including an 50%
H
2
SO
4
(20 mmol) at 60 ºC for 170 h under 6.5 MPa of H . The MeOH yield (based on the
2
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multiplication of the conversion of FA and MeOH selectivity) reached 43%, which was the
4
1
highest MeOH %-yield among the previous reports (entry 2). We have also isolated the
MeOH/water mixture from the reaction mixture (entry 5). The reaction of 4 M FA solution
10
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48
49
50
51
52
53
54
55
56
57
58
59
60
including 50% H
resulted in the almost all conversions of FA and the production of 14.2 mmol of MeOH (MeOH
selectivity of 35.7%) as judged by GC and HPLC. The produced MeOH and H O were distilled
out under reduced pressure to give 5.9 mL of colorless liquid. The collected volatiles were
reanalyzed by GC and HPLC contained 2.50 M of MeOH and 7.9 mM of HCO Me, indicating
4.8 mmol of MeOH and HCO Me mixture in solution. The similarity of the values between the
2
SO
4
(20.0 mmol, 2 M as solution) at 60 ºC for 65 h under 6.0 MPa of H
2
2
2
1
2
crude mixture and the isolate liquid should afford the responsible MeOH yield in our
experiments to some extent.
Mechanistic Insight by Isotope Effect, NMR Study
As mentioned above, iridium complexes catalyzed both the dehydrogenation and the
hydrogenation of FA. The FA dehydrogenation generally progress in three steps in Cp*Ir
complexes bearing a N,N’-bidentate ligand: (i) formation of formato complex ([Ir]-OCOH), (ii)
release of CO
2
by β-hydrogen elimination to generate iridium-hydride complex ([Ir]-H), and (iii
from the reaction of [Ir]−H and a proton (Scheme 2).54,55 On the other
and iv) production of H
2
hand, the proposed mechanism of the FA hydrogenation that (vii) [Ir]−H reduces the protonated
FA acid via hydride transfer to give formaldehyde followed by (ix) the further reduction to give
4
0
MeOH. To gain the further insight into the overall reaction process for 4b including the resting
state of the catalytic reaction, we carried out isotope effect and NMR studies.
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2
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2
3
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4
4
4
4
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5
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5
5
5
5
5
5
6
+
2
N
N
Cp*
Ir
H O
CH OH
2
3
[
Ir]-OH
2
[
Ir]-H
+HCO H
2
+
+
–H
+
(ix)
H
(iv)
(v)
(
i)
H
+H
–H
2
2
+
H O –H O
2 2
O
0
1
2
3
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5
6
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8
9
0
1
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6
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0
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0
1
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3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
+
2
H
Cp*
O
N
N
N
N
Cp*
Ir
Ir
H
H
H
(vii)
O
X
[Ir]-H
2
( )
H
[Ir]-OCOH
O
(
iii)
(vi)
–H
(ii)
+
+
H
OH
viii)
+H
– CO2
Me
(
+
+
H
Cp*
H
N
N
Ir
N
N
N
=
O
[
Ir]-H
N
H
OH
Me
Scheme 2. Proposed Mechanism of FA hydrogenation to MeOH by Using 4b ([Ir]-OH ).
2
At first, an isotope effect of the reaction was studied under the atmospheric conditions with 1 M
FA including 5 mol% H SO (0.5 mmol) at 60 °C. When FA or water/H SO were replaced with
2
4
2
4
deuterated reagents, the conversion of FA decreased considerably, which was consistent with the
previous report (Table 5).40,54,55 Interestingly, the MeOH selectivity without H
2
in D
2
O/D
2
SO of
4
4.0-4.5% was almost twice as large as that in H
2
O/H
2
SO
4
of 2.1% (entries 1, 2 vs 3, 4). The
(or HD, D
enhancement of the selectivity would be caused by the suppression of the H
2
2
)
generation via (iii and iv) the protonation of Ir−hydride (or Ir-deuteride), which will be a rate
determining step in FA dehydrogenation.54,55 In contrast, it seems that the hydride transfer to FA
(
vii) was insignificantly affected by deuteration. However, the further investigation would be
necessary to conclude precisely. Furthermore, the reaction of 4 M DCO D in D O (10 mL, 40
mmol) with 5% D SO (2 mmol) for 6 h gave 103 µmol of MeOH with 15.4% conversion of FA
entry 5). Under the atmospheric conditions (i.e., the reaction was carried out without
2
2
2
4
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introduction of external H
2
in unclosed system), the MeOH selectivity of 15.4% was a
remarkable value in conversion of FA to MeOH. The results imply that the FA hydrogenation
opened up the possibility of MeOH production under mild reaction conditions.
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Next, NMR studies on the reaction of 4b with FA in the presence or absence of sulfuric acid
were conducted. When FA (200 µmol, 40 equiv.) was added to a D O solution of 4b (5 µmol) in
2
the absence of sulfuric acid, the corresponding Ir-hydride complex was observed as a major
1
product (Figure S11a). The H NMR signals except for the iridium-hydride were identical to
those obtained by the reaction of 4b and sodium formate (Figure S13). The chemical shift of the
corresponding iridium-hydride (−11.48 ppm) at neutral conditions was comparable to that of the
+
54,60
analog [Cp*Ir(bpy)(H)] (−11.11 and −11.80 ppm).
The disappearance of iridium-hydride
signal was due to the fast H/D exchange under acidic conditions and the formation of Ir-
deuteride ([Ir]-D).60,61 After heating the FA solution at 50 ºC for 20 h, [Ir]-D still existed as a
major product (Figures 7a and S11c). This result indicates that [Ir]-D is the resting state of the
catalytic cycle.
In contrast, the starting complex 4b remained as a major complex in the presence of D
µmol, 30 equiv., Figure S12), which may be explained that the externally added deuteron
SO ) prevented the existence of hydride/deuteride complex. Heating the solution under the
same conditions (50 ºC, 20 h) gave the mixture of 4b and the new complex X in a ratio of 1/0.49
as judged by the integral of a Cp* moiety). Coinstantaneously, the production of MeOH (12.3
µmol) was definitely observed in the presence of D SO (Table S6). The produced MeOH was
DOD (8%)), due to facile H/D exchange of iridium-
2
SO
4
(150
(D
2
4
(
2
4
dueterized (i.e., CHD
2
OD (92%) and CH
2
deuteride [Ir]-D in D
2
O (Figure 7b).30,40,60,61
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2
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2
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Table 5. Isotope Effect in FA Disproportionation under Atmospheric Pressurea
Entry Substrate / Solvent
Yield
MeOH
of MeOH
selectivity
TOF for FA FA Conv.
e
dehydrogenation (%)
b
b,c
−1 d
(
µmol)
without H
2
(h )
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5f
HCO
DCO
HCO
DCO
DCO
DCO
2
2
2
2
2
2
H/H
D/H
H/D
D/D
D/D
D/D
2
2
2
2
2
2
O+H
O+H
O+D
O+D
O+D
O+D
2
2
2
2
2
2
SO
SO
SO
SO
SO
SO
4
4
4
4
4
4
38.1
24.8
40.0
32.5
103
2.2
118
75
75
50
25
-
52
2.0
37
4.0
30
4.5
22
15.4
15.3
5
6fg
291
14
a
Reaction conditions: 1 M FA (10 mL, 10 mmol) in water including 5% H
2
SO
4
(0.5 mmol) and
b
c
catalyst 4b (10 µmol) at 60 ºC for 6 h. Sum of MeOH and HCO
without H is calculated from the mole of the produced MeOH and HCO
third of the mole of the consumed FA. Average TOF over the initial 10 min of dehydrogenation
of FA. Determined by HPLC. 4 M DCO
was used instead of 1 M FA including 5% H
2
Me was used. MeOH selectivity
2
2
Me divided by the one
d
e
f
2
D (10 mL, 40 mmol) including 5% D
2
SO (2 mmol)
4
g
2
SO for 6 h. for 24 h.
4
b)
Cp* region
CHD OD
2
4b
[Ir]-D
2
X
(
)
CH DOD
2
(X10)
a)
[Ir]-D
(X10)
(ppm)
1
Figure 7. H NMR spectra of the crude mixture of 4b and of HCO
2
H (40 equiv.) after heating at
50 ºC for 20 h in a sealed tube under atmospheric pressure in a) D
2
O and b) 0.25 M D SO /D O.
2
4
2
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In order to identify the characteristics of the new complex X, NMR studies using C-formate
1
3
were conducted firstly. The reaction of 4b with 22 equiv. H CO
2
Na in D O followed by the
2
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13
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15
16
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addition of 40 equiv. of D SO gave the mixture of 4b, [Ir]-D, and X in a ratio of 1/0.3/0.7
2
4
1
3
(Figure S14). A singlet signal assignable to OCOH was not observed from 6 to 10 ppm. The C
13
NMR of the mixture showed no signals except for those assignable to H CO
2
H (δ = 168.1) and
C
13CO
(δ
C
= 127.1) (Figure S15). Further addition of D
SO (80 equiv.) led to a decrease of both
2
2
4
[Ir]-D and X, and consequently 4b became a major complex. In the mass spectrum of the
mixture including 4b, [Ir]-D, and X, the signal of 557 assignable to [Ir]-OCOH was not
observed (Figure S18).
Next, complex 4b in the absence of sulfuric acid was pressurized with 2.0 MPa of H for 8 h, the
2
mixture of 4b, [Ir]-D, and X in a ratio of 1/1.1/0.6 (Figure S16). No signal was observed in the
region of high magnetic field, due to the fast H/D exchange in acidic conditions.60,61 Meanwhile,
the 4b remained as a major complex together with approximately 10% of X for 8 h in the
presence of D
2
SO
4
(Figure S17). The complex 4b and X may be favourable to [Ir]-D in the
gas. From these results, it is clear that X was
acidic conditions even in the pressurization of H
2
ruled completely out formate complex [Ir]-OCOH. Currently, we believe that complex X may
.37,62,63
be σ-coordinated dihydrogen/dideuteron complex [Ir]-H
2
/D
2
On the basis on the results, the FA hydrogenation by using 4b is summarized as follows (Scheme
2): 1) Iridium hydride [Ir]-H is generated from both FA (paths: i and ii) and H (paths: v and vi),
where the formation of [Ir]-H from FA may be faster than from H . The increase of the
2
2
pressurization of H
2
might work favourably for the suppression of the generation of H (paths: iii
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and iv) to some extent, which makes [Ir]-H and/or [Ir]-H the stable species under the acidic
2
conditions. 2) The addition of H
stage of the addition of H SO , the protonation of [Ir]-H and a hydride transfer to FA may
accelerate (paths iii and vii). The further increase of H SO should suppress the formation of
[Ir]-H via [Ir]-OCOH (path: i → ii). Other influence of H
among [Ir]-H, [Ir]-H
equilibrium should shift to [Ir]-H at low concentration of H
2
SO changes the reaction rates in the catalytic cycle. At an early
4
2
4
0
1
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1
2
3
4
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9
0
2
4
2
SO
4
is the change of the resting state
SO . That is, the possible
and to 4b at high concentration.
2
and 4b in response to the concentration of H
2
4
2
SO
4
The most important role of H
2
SO
4
is the protonation of FA (path viii), which increases the
37
reactivity for a hydride transfer of [Ir]-H. Therefore, the relatively long-term stability of [Ir]-H
in the acidic conditions and the low activation energy of a hydride transfer should attribute to the
high yield and selectivity of MeOH in this system.64
Conclusion
After the comprehensive investigation on 2,2′-bipyridine derivatives and its reaction conditions,
we have demonstrated the highly efficient FA hydrogenation to MeOH by using the catalyst
(
4b), which was tuned its electronic-property, and adding H
2
SO4. The high hydrogen pressure,
the high concentration of H SO , and the high initial FA concentration were all contributed to the
2
4
increase to the MeOH yield and selectivity. Finally, the TON of 1314 (3.9 M MeOH solution)
and the MeOH selectivity up to 47.1%, respectively, were achieved. The NMR study revealed
that equilibrium between dihydrogen complex and hydride complex was affected by
concentration of H
2
SO . Especially, the relatively long-term stability of hydride complex in the
4
acidic conditions and the low activation energy of hydride transfer derived from the protonated
FA may attribute to the high yield and selectivity of MeOH in hydride complex. Interestingly,
1
5.4% of MeOH selectivity was obtained even under the atmospheric pressure in D O, probably
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due to inhibition of the H
2
or D
2
production by protonation of deuteride complex as a
competitive reaction. These results provide the productive information on the catalyst design and
4
3
the catalytic system of the FA hydrogenation. The combination of CO
2
hydrogenation and this
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60
FA hydrogenation may open up the possibility of MeOH production from CO
2
under low
temperature and atmospheric pressure.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Authors
Yuichiro Himeda, himeda.y@aist.go.jp
Gábor Laurenczy, gabor.laurenczy@epfl.ch
Present Addresses
§Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai,
Osaka 599-8531, Japan
Supporting Information. Experimental section including general methods, synthesis and NMR
spectra of 4b and 5, the procedures for the hydrogenation reaction under pressurization and
atmospheric pressure, and NMR studies. This material is available free of charge via the Internet
at http://pubs.acs.org.”
ACKNOWLEDGMENT
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Y. H. and N.O. thanks the Japan Science and Technology Agency (JST), ACT-C for financial
support. K.S. and G.L. are grateful to the Swiss National Science Foundation and EPFL for
financial support. We also thank to Dr. Hide Kambayashi in AIST, Japan, for the measurement
of NMR and MS.
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(
48) During the preparation of this manuscript, the group of Kubiak reported the
III
electronic effects on the FA disproportionation to MeOH by [Cp*Ir (R-bpy)Cl]Cl complexes (R
, H, Me, tert-Bu, OMe), where substituents (R) were introduced at 4,4′-position of 2,2′-
bipyridine. The unsubstituted bipyridine complex showed the highest methanol selectivity of
=
CF
3
1
.17% for 3 M FA aqueous solution at 60 ºC for 21 h.
(
(
49) Sasayama, A. F.; Moore, C. E.; Kubiak, C. P. Dalton Trans. 2016, 45, 2436-2439.
50) The reaction mixtures including greater than 0.4 M H SO solution (greater than
SO in 4 M FA solution) were diluted before the GC analysis due to the analytical
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
4
1
0 mol% H
2
4
limitation (See Supporting Information) As a result, the ratio of MeOH and methyl formate
significantly changed along with the shift of the equilibrium.
(
(
(
51) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165-195.
52) Himeda, Y. Green Chem. 2009, 11, 2018-2022.
53) Suna, Y.; Ertem, M. Z.; Wang, W.-H.; Kambayashi, H.; Manaka, Y.; Muckerman,
J. T.; Fujita, E.; Himeda, Y. Organometallics 2014, 33, 6519-6530.
(54) Wang, W.-H.; Xu, S.; Manaka, Y.; Suna, Y.; Kambayashi, H.; Muckerman, J. T.;
Fujita, E.; Himeda, Y. ChemSusChem 2014, 7, 1976-1983.
(55) Ertem, M. Z.; Himeda, Y.; Fujita, E.; Muckerman, J. T. ACS Catal. 2016, 6, 600-
6
09.
(56) The coordination of sulfate anion to the iridium center might be possible. The
experiment of the addition of Na
between aqua complex 4b and sulfate complex in solution. See Figure S2 in the Supporting
Information.
2
SO
4
into a D O solution of 4b indicated the equilibrium
2
(57) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33, 4453-4459.
(58) Wang, W.-H.; Ertem, M. Z.; Xu, S.; Onishi, N.; Manaka, Y.; Suna, Y.;
Kambayashi, H.; Muckerman, J. T.; Fujita, E.; Himeda, Y. ACS Catal. 2015, 5, 5496-5504.
(59) Onishi, N.; Ertem, M. Z.; Xu, S.; Tsurusaki, A.; Manaka, Y.; Muckerman, J. T.;
Fujita, E.; Himeda, Y. Catal. Sci. Technol. 2016, 6, 988-992.
(
60) Abura, T.; Ogo, S.; Watanabe, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125,
149-4154.
61) Wang, W.-H.; Hull, J. F.; Muckerman, J. T.; Fujita, E.; Hirose, T.; Himeda, Y.
4
(
Chem. Eur. J. 2012, 18, 9397-9404.
(62) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledós, A.; Pons, V.; Heinekey, D. M. J.
Am. Chem. Soc. 2004, 126, 8813-8822.
(63) Vogt, M.; Pons, V.; Heinekey, D. M. Organometallics 2005, 24, 1832-1836.
(64) We believed that the equilibrium shifts from methyl formate to FA and MeOH
along with the consumption of FA. However, the direct hydrogenation of methyl formate and/or
protonated methyl formate cannot be completely excluded at this stage based on the previous
report on the hydrogenation of ester to give methanol. See refs 21, 22, and 26.
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